There may be various ways to achieve what you are trying to do, but one of the most straight forward ways is to employ two comparators on your input that detects voltages "outside" the logic threshold zone of your logic gate input. Say, if one comparator detects a level < 0.7xVcc and the second comparator detects an input voltage > 0.3xVcc then you consider your input NOT to be of a logic level, and rather some "Z", otherwise it IS a "true" logic level and everything stays normal. When this "other" condition is met then you switch off your Vcc to the LEDs to keep them off. There may be other simpler ways or less costly, to go about it, but the principle is that that your "Z" condition is that zone of voltages that does not normally correspond to your expected logic level thresholds. The "Z" input condition will require setting a suitable bias input voltage (say 0.4xVcc or other similar), but this will be driven true high/low when ever there is a true logic input. You will of course need to keep your logic from getting in a saturated state, but this you may need to figure out the best way to do ... Hope this helps to get you going.

There may be various ways to achieve what you are trying to do, but one of the most straight forward ways is to employ two comparators on your input that detects voltages "outside" the logic threshold zone of your logic gate input. Say, if one comparator detects a level < 0.7xVcc and the second comparator detects an input voltage > 0.3xVcc then you consider your input NOT to be of a logic level, and rather some "Z", otherwise it IS a "true" logic level and everything stays normal. When this "other" condition is met then you switch off your Vcc to the LEDs to keep them off. There may be other simpler ways or less costly, to go about it, but the principle is that that your "Z" condition is that zone of voltages that does not normally correspond to your expected logic level thresholds. The "Z" input condition will require setting a suitable bias input voltage (say 0.4xVcc or other similar), but this will be driven true high/low when ever there is a true logic input. You will of course need to keep your logic from getting in a saturated state (or eliminate the logic gate completely), but this you may need to figure out the best way to do ... Hope this helps to get you going.

There may be various ways to achieve what you are trying to do, but one of the most straight forward ways is to employ two comparators on your input that detects voltages "outside" the logic threshold zone of your logic gate input. Say, if one comparator detects a level < 0.7xVcc and the second comparator detects an input voltage > 0.3xVcc then you consider your input NOT to be of a logic level, and rather some "Z", otherwise it IS a "true" logic level and everything stays normal. When this "other" condition is met then you switch off your Vcc to the LEDs to keep them off. There may be other simpler ways or less costly, to go about it, but the principle is that that your "Z" condition is that zone of voltages that does not normally correspond to your expected logic level thresholds. Hope this helps to get you going.

There may be various ways to achieve what you are trying to do, but one of the most straight forward ways is to employ two comparators on your input that detects voltages "outside" the logic threshold zone of your logic gate input. Say, if one comparator detects a level < 0.7xVcc and the second comparator detects an input voltage > 0.3xVcc then you consider your input NOT to be of a logic level, and rather some "Z", otherwise it IS a "true" logic level and everything stays normal. When this "other" condition is met then you switch off your Vcc to the LEDs to keep them off. There may be other simpler ways or less costly, to go about it, but the principle is that that your "Z" condition is that zone of voltages that does not normally correspond to your expected logic level thresholds. The "Z" input condition will require setting a suitable bias input voltage (say 0.4xVcc or other similar), but this will be driven true high/low when ever there is a true logic input. You will of course need to keep your logic from getting in a saturated state, but this you may need to figure out the best way to do ... Hope this helps to get you going.